Growth of polycrystalline CaF2 via low temperature OMCVD
Research relating to the development of the present invention was partially supported by Contract No. N00014-87-K-0323 from the U.S. Office of Naval Research. 
    
    
     BACKGROUND 
     The present invention relates to a method for manufacturing a silicon-on-insulator (&#34;SOI&#34;) structure based on a layer of calcium fluoride (CaF 2 ) as the insulator. 
     The development of SOI material structures are of considerable interest in integrated circuit technology because successful development of such structures will allow fabrication of three-dimensional integrated circuits. Additionally, SOI material structures will allow fabrication of memory and logic circuits that are immune to both soft errors due to alpha particles and single particle upset events caused by high-energy particles (1). 
     SOI structures are being pursued as a technique to isolate each device in a chip on its own silicon mesa, which would sit on top of an insulating layer. As a result, less chip area would be required than the present isolation techniques in standard silicon technology. &#34;Latch up&#34; would be eliminated and stray captive coupling would also be reduced. Currently, most research and development activity in SOI focuses on the use of a layer of amorphous SiO 2  with the thickness of 1500-5000 Å as the insulator because of the excellent properties of SiO 2  and its excellent compatibility with silicon. The single crystal layer of silicon over the SiO 2  is formed generally by two main techniques involving recrystalizing amorphous or polycrystaline silicon from seed locations in the underlying silicon substrate, or implanting a very high dose, approximately 10 18  ions/cm, ion implantation of oxygen, to form an amorphous SiO 2  layer below the surface of silicon substrate. 
     Although prior techniques have met with moderate success, difficulty exists in obtaining low defect density, high-quality single crystal silicon on amorphous SiO 2  layers. An alternative approach to the formation of the SOI structure is utilizing a single-crystal, lattice-matched insulator, which allows the possibility for growth of a single crystal overlying silicon layer directly on the insulator. 
     Calcium fluoride (CaF 2 ) is optimal for fabricating SOI because of its wide band gap and a cubic structure similar to that of silicon. CaF 2  is lattice matched to silicon to within 0.6% at room temperature. 
     SOI insulators based on CaF 2  have been demonstrated using molecular beam epitaxy (MBE) (2), but more work is needed to achieve high electrical quality SOI structures and interfaces, and topographically smooth layers of uniform thickness. Using MBE, CaF 2  is grown using CaF 2 , evaporated from tungsten or graphite crucibles at very high temperatures (1200-1400° C.) onto silicon substrates at temperatures from 550-700° C. (3,4). 
     Chemical vapor deposition (&#34;CVD&#34;) was used in some of the earlier efforts to grow silicon insulator (SI) on insulating substrates, including CaF 2 . However, the gases involved in the CVD process were subject to a growth temperature of about 1100° C. Additionally, the gases reacted with the CaF 2  substrate, resulting in poor epitaxy. 
     One key requirement for the success of the present approach has been to identify and develop appropriate volatile organometallic precursors and CVD reations which will produce CaF 2 . Unlike other well documented organometallic chemical vapor deposition (OMCVD) processes such as those employed for the fabrication of thin films of III/V compound semiconducts (5) there appear to be no previous reports of CVD growth of CaF 2 . This may be due to the lack of suitable volatile calcium compounds (6). 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide an improved method for fabricating a SOI structure utilizing CaF 2  as the insulator. This improved method should, because lower temperatures are utilized, help prevent the problems of layer mixing, wafer warpage, and film stresses associated with high temperature depositions. This improved method is based on an organometallic chemical vapor deposition (OMCVD) reaction to form CaF 2 . The chemical vapor deposition of CaF 2  on a substrate is accomplished by placing an organocalcium compound and a gas which is a source of fluorine in a reactor which allows gases ready access to the substrate. Next, the reactor is heated until CaF 2  appears on the substrate. 
     The present invention involves a method for deposition of CaF 2  on a substrate, the substrate being preferably silicon. This method comprises the following steps. Initially a sealable system is prepared which includes a reactor with a heatable portion and a substrate. An organocalcium compound is placed in the reactor and a gas which is a source of fluorine is fed into the system. After sealing the system, the reactor is heated until the substrate is coated with calcium fluoride. The reactor preferably has an organocalcium compound-containing portion which may be heated. The substrate is in part of the system in communication with the heatable portion so that vaporized organocalcium compound and fluorine-containing gas have access to the substrate. 
     A preferred organocalcium compound is bis-pentamethylcyclopentadienyl calcium and a preferred gas which is a source of fluorine is SiF 4  or NF 3 . For the introduction of the gas which is a source of fluorine, the placing step preferably involves a reduction of pressure in the system to at least about 10 -2  Torr before introduction of the said gas. The gas which is a source of fluorine is preferably added until pressure inside the system reaches about 10 Torr. 
     The heating step generally involves heating the reactor to a temperature between about 100° C. and about 550° C., preferably between about 200° C. and about 400° C., more preferably between about 150° C. and about 200° C. and most preferably about 150° C. These variations in temperature most usually relate to the vaporization temperatures of the volatile organocalcium compound being used although they may also be varied to control the rate of vaporization and CaF 2  deposition on the substrate. A preferred reactor for this system is a vertical or horizontal hotwall glass reactor containing a substrate, which allows gaseous substances to contact the substrate. In the above-described heating step the reactor preferably has a lower portion which contains the organocalcium compound and is heated, preferably for at least about one hour, which allows formation of adequate amounts of CaF 2  on the substrate surface. 
     In a particularly detailed embodiment, the method of the present invention for preparing a silicon substrate coated with CaF 2  may be described as comprising the following steps: (1) preparing or otherwise obtaining an organocalcium compound; (2) preparing or otherwise obtaining a gas which is a source of fluorine; (3) placing the organocalcium compound in a heatable portion of a reactor and introducing the gas which is a source of fluorine into the reactor system, said reactor system containing a second portion comprising the substrate; (4) sealing the reactor system; (5) heating the heatable portion of the reactor until said substrate is coated with CaF 2  ; and (5) removing coated silicon substrate from the reactor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     These theories are presented as illucidating and are not meant to represent limitations of the invention unless otherwise specifically indicated herein. 
     &#34;Me&#34; is used to represent a methyl group. The term &#34;in communication&#34; is defined as a mechanism that will allow liberated gases from a reactor ready access to a substrate. 
     Some features of the present invention are: (1) the first chemical vapor deposition (CVD) of CaF 2  from volatile precursors and (2) ultra low temperature formation of thin film (polycrystalline) CaF 2  using this technique. CaF 2  is an excellent choice for the insulator in SOI structures because of its wide band gap and its lattice matching to silicon within 0.6%. 
     The present invention involves the use of organocalcium precursors for the chemical vapor deposition of thin CaF 2  films under exceptionally mild conditions. This method is based on utilizing an organocalcium chemical vapor deposition reaction to form CaF 2 . 
     Earlier work on organocalcium chemistry focused on utilizing small ligands such as methyl and ethyl. The lack of steric saturation often resulted in intractable oligomeric materials or decomposition products. The use of bulkier groups results in isolation of stable organocalcium derivatives. 
     Bis(cyclopentadienyl) calcium is polymeric in the solid state, but the volatile organometallic derivative bispentamethylcyclopentadienyl calcium (C 5  Me 5 ) 2  Ca is monomeric and may be sublimed at 75° C. and 1× 10 -3  Torr pressure. This compound was recently reported by Burns et al. (7). The present invention involves low temperature CVD of CaF 2  using bis-pentamethylcyclopentadienyl calcium with either SiF 4  or NF 3  as the source of fluorine. The overall process involves the reaction of a volatile organometallic source of calcium with a gaseous source of fluorine. 
     The overall reaction stochiometries appear to be: 
     
         (C.sub.5 Me.sub.5).sub.2 Ca+2SiF.sub.4 →CaF.sub.2 +2C.sub.5 Me.sub.5 SiF.sub.3 or 
    
     
         (C.sub.2 Me.sub.5).sub.2 Ca+2NF.sub.3 →CaF.sub.2 +2C.sub.5 Me.sub.5 NF.sub.2 
    
     This is the first chemical vapor depositions of calcium fluoride (CaF 2 ) from volatile precursors at ultra low temperatures in the formation of thin film calcium fluoride. The temperature in Example I is, for example, 150° C. In comparison, calcium fluoride has been previously used in MBE growth by evaporation of calcium fluoride CaF 2  from tungsten or graphite crucibles at high temperatures of 1200-1400° C. The lower temperature of 150° C. in Example I is desirable to prevent difficulties such as layer mixing, wafer warpage and film stresses that plague high temperature depositions. The growth of polycrystaline calcium fluoride CaF 2  at 150° C. is a remarkably low temperature for the growth of calcium fluoride, compared to other techniques. 
     NaC 5  Me 5  is made from pentamethylcyclopentadiene by the reaction of Na (dispersion) with pentamethylcyclopentadiene in THF. Pentamethylcyclopentadiene can be obtained from the Aldrich Chemical Company, Milwaukee, Wisconsin. 
     Bis-pentamethylcyclopentadienyl calcium is prepared from CaI 2  and NaC 5  Me 5  by the reactions: ##STR1## 
     EXAMPLE I 
     A vertical hot-wall glass reactor was connected to a vacuum manifold which permits the introduction of SiF 4  or NF 3 . The organometallic calcium compound was placed in the lower portion of the tube. The system was then evacuated to about 10 -2  Torr. SiF 4  was added until the pressure reached 10 Torr. The system was sealed and partially submerged in an oil bath maintained at 150° C. After one hour, white polycrystalline deposits were observed on the walls of the vessel in the region immediately above the oil level. 
     Polycrystalline deposition may be obtained under similar conditions on glass and Si(100). Growth rates and film thicknesses appear to be influenced by the distance of the Cp* 2  Ca from the reaction regime in the reactor. Thus, thicknesses varying from 10 to 100 μm can be observed. These correspond to very approximate growth rates of 10 to 100 μm h -1 . There appeared to be little difference in the film properties when grown on glass or silicon substrates or when using either SiF 4  or NF 3 . X-ray powder diffraction patterns matched the JCPDS data and confirmed that the deposits were polycrystalline CaF 2 . 8  Nomarsky optical microscopy confirmed the polycrystalline nature of the product. The size of the crystallites appeared to range from 20 to 50 μm. XPS data were obtained on various samples which were etched twice with ionized argon at 5 minute intervals. 9  The corrected Ca 2p and F ls signals at 347.7 and 684.6 eV respectively compared well with the literature values (347-348, 689.6 eV), and confirm the Ca:F ratio of 1:2. XPS analysis also revealed the presence of considerable quantities of carbon (18%) and oxygen (7%) in the films. This is not altogether unexpected since no particular precautions were taken in handling the samples. The high carbon levels may also be due to organic fragments which remain in the deposited material. 
     Furthermore, although the CaF 2  produced by the method was polycrystalline in nature, it is believed that this is a major step toward the goal of low temperature growth of epitaxial CaF 2  on silicon, using the method of the present invention. 
     The following literature references in pertinent part are incorporated by reference herein for the reasons cited. 
     REFERENCES 
     (1) Lam, et al., VLSI Electronics Microstructure Science 1982, 4, 1. 
     (2) Schowalter, et al., J. Vac. Sci. Technol. 1986, A4, 1026. 
     (3) Schowalter, et al., J. Appl. Phys. 1985, 58, 302. 
     (4) Reichelt, et al., J. Vac. Sci. Technol. 1973, 10, 1153. 
     (5) Ludowise, J. Appl. Phys., 1985, 58, 231. 
     (6) Lindsell, Comprehensive Organometallic Chemistry 1982, Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Eds, Vol. 1, Pergamon Press. 
     (7) Burns, et al., J. Organomet. Chem. 1987, 325, 31 and references therein. 
     (8) Joint Committee for Powder Diffraction Standards (JCPDS), International Center for Diffraction Data, 1601 Park Lane, Swarthmore, PA 19081, Card No. 35-816, 1988. 
     (9) XPS data were obtained on a VG Scientific Escalab Mark II with a Mg(K-α)X-ray anode. The observed binding energies [Ca 2P (354.1leV),F ls (691.OeV)] were corrected for electrostatic charging. Literature values were obtained from &#34;The Handbook of X-ray Photoelectron Spectroscopy&#34;, Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Muilenberg, G. E. G. E. (eds) Perkin-Elmer, Physical Electronics Division, 1976. 
     Changes may be made in the particular organocalcium compounds, gaseous sources of fluorine, temperatures and pressure or in the steps of the method described herein without departing from the concept or scope of the invention as defined in the following claims.